The present invention relates to position sensorless permanent magnet motor drives and in particular to a method of sensing rotor position in such motors using stator-based saliency (sometimes referred to as motor asymmetry).
Permanent magnet synchronous motors provide a torque through the interaction of a magnetic field generated by one or more electrical coils and one or more permanent magnet elements. The magnetic field generated by the current supplied to the coils must be synchronized with the position of the magnetic field of the permanent magnets and consequently such motors are normally termed “synchronous motors”.
Permanent magnet synchronous motors place the permanent magnets on the rotor to provide multiple, angularly spaced magnetic poles (eliminating the need for brushes to carry power to the rotating rotor) and sense rotor position to properly synchronize the drive current to coils in the stator.
In a “surface permanent magnet synchronous motor” (SPMSM), the permanent magnets of the rotor are on the surface of the rotor. In an “interior permanent magnet synchronous motor” (IPMSM), the permanent magnets are embedded in a metallic core of the rotor.
The sensing of rotor position, in order to properly coordinate the drive signals to the coils of the stator, may be accomplished by a variety of means. For example, one or more Hall effect sensors may be placed on the stator to sense changes in the magnetic field generated by the rotor as it spins. Alternatively, position resolution may be obtained by mounting an encoder or a resolver to the rotor shaft. Absolute rotor position is not necessary but only relative position of the motor poles with respect to the stator coils.
Discrete rotor position sensors may substantially decrease the reliability of the motor by introducing additional electrical conductors and connectors that are substantially less robust than the motor windings and their terminations. Accordingly there is considerable interest in producing “position sensorless” motor drives, for example, by analyzing slight perturbations in the drive currents caused by motor saliency. Saliency, as used herein, refers to periodic fluctuations in the electrical properties of the motor as a function of the position of the rotor. These saliency-based sensorless control implementations are described in U.S. Patent Publication Nos. US 1996/5565752, US 2000/6069467, US 2006/7045988 B2 and US 2006/7098623 B2 all hereby incorporated by reference.
One type of motor saliency that may be exploited for this purpose is the change in reluctance between stator coils caused by the permanent magnet inclusions in the rotor in an IPMSM design. As the rotor moves and the permanent magnet inclusions move into and out of an inter-coil path, this change in reluctance may be detected by a carrier current superimposed on the drive signal of the stator. IPMSM sensorless drive is reported in “Ogasawara et al., Implementation and position control performance of a position sensorless IPM motor drive system based on magnetic saliency,” Industry Applications, IEEE Transactions on, vol. 34, pp. 806-812, 1998 also hereby incorporated by reference.
In a SPMSM design, small amounts of exposed steel in between the magnets may saturate at particular rotor positions. Again, a high frequency injection signal may be used to detect this saturation which will result in a modulation of a measured induction of the stator coils. SPMSM sensorless drive is described in “Jang et al., Sensorless drive of surface-mounted permanent-magnet motor by high-frequency signal injection based on magnetic saliency,” Industry Applications, IEEE Transactions on, vol. 39, pp. 1031-1039, 2003 also hereby incorporated by reference.
Current position sensorless detection systems are relatively ineffective in SPMSM designs which provide a substantially continuous outer magnetic surface. Many position sensorless systems are susceptible to interference by the drive signal which may obscure sensitive measurements of inductance or reluctance.